The spine is a series of individual bones called vertebrae, separated by cartilaginous disks. The spine includes seven cervical (neck) vertebrae, twelve thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, and the fused vertebrae in the sacrum and coccyx that help to form the hip region. While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow tube containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae. The spinal disks act as shock absorbers, cushioning the spine, and preventing individual bones from contacting each other. Disks also help to hold the vertebrae together. The weight of the upper body is transferred through the spine to the hips and the legs. The spine is held upright through the work of the back muscles, which are attached to the vertebrae. While the normal spine has no side-to-side curve and rotational misalignment, it does have a series of front-to-back curves, giving it a gentle “S” shape. If the proper shaping and/or curvature are not present due to scoliosis, neuromuscular disease, cerebral palsy, or other disorder, it may be necessary to straighten or adjust the spine into a proper curvature and alignment.
Generally the correct curvature and alignment are obtained by manipulating the vertebrae into their proper position and securing that position with screws and rods. The rods which are shaped to mimic the correct curvature and force the spine into proper alignment. Bone grafts are then placed between the vertebrae to aid in fusion of the individual vertebrae together to form a correctly aligned spine.
Spinal deformity correction procedures can require complex anatomical manipulation to restore proper anatomic form to the patient. Currently, some systems have non-linkable tubes with handles to perform vertebral column manipulation (VCM). VCM with a rotational force applied on bone anchors in the coronal plane (medial-laterally) is referred to as “derotation”. Because correction of a large, complex 3-dimensional spinal deformity can exert a high stress concentration on the bone anchor element and anatomy, forces should be distributed between multiple levels and multiple bone anchors in order to reduce the occurrence of anatomical damage (i.e. screw breach through the vertebrae).